Due to the increasing popularity of terminals or Mobile Stations (MSs) such as smart phones, the average amount of data consumed by mobile users has exponentially increased, and the users' demands for higher data rates have also constantly increased.
Generally, a method for providing a high data rate in a mobile communication system may be divided into a method of using a wider frequency band to provide communication, and a method of increasing the frequency use efficiency. It is very difficult to provide a higher average data rate with the latter method, because the communication technologies of the current generation already support the frequency use efficiency close to its theoretical limit, making it difficult to further increase the frequency use efficiency by improving the technologies.
Therefore, a more feasible way to increase the data rate is to provide data services over a wider frequency band. In this case, available frequency bands need to be considered. In the current frequency distribution policy, available broadband communication bands of 1 GHz or more are limited, and the available frequency bands include only the millimeter wave (mmW) bands of 30 GHz or more. In these high frequency bands, unlike in the 2 GHz band used by the conventional cellular systems, signals are significantly attenuated depending on the distance. Due to the signal attenuation, in the case of a BS that uses the same power as that of the conventional cellular system, its coverage may be significantly decreased. In order to solve these and other problems and disadvantages, beamforming techniques are widely used to increase the transmission/reception efficiency of an antenna by concentrating transmit/receive power in a narrow space.
FIG. 1 illustrates a mobile communication system including a MS and a BS that provides beamforming using array antennas.
Referring to FIG. 1, a BS 110 may transmit data in each of cells (or sectors) 101, 103 and 105 using a plurality of array antennas Array0 and Array1 by switching the direction of a Downlink (DL) Transmit (Tx) beam 111. A MS 130 may also receive the data by switching the direction of a Receive (Rx) beam 131.
In the mobile communication system that performs communication using the beamforming technique, the BS 110 and the MS 130 provide data services by selecting the direction of a Tx beam and the direction of a Rx beam, which show the optimal channel environment, from among a variety of directions of the Tx beam and the Rx beam. The beamforming technique may be equally applied not only to a DL channel carrying data from the BS 110 to the MS 130, but also to an Uplink (UL) channel carrying data from the MS 130 to the BS 110.
In the beamforming technique, if it is assumed that the number of directions of a Tx beam, in which the BS 110 can transmit data, is N, and the number of directions of a Rx beam, in which the MS 130 can receive data, is M, the simplest way to select the optimal DL Tx/Rx direction is that the BS 110 transmits a predetermined signal in each of N available Tx beam directions at least M times, and the MS 130 receives each of N Tx beams using M Rx beams. In this method, the BS 110 transmits a specific reference signal at least N×M times, and the MS 130 receives the reference signal N×M times and measures signal strength of the received reference signal. The MS 130 may determine, as the optimal Tx/Rx beam direction, the direction that shows the highest measured signal strength among the N×M measured signal strengths.
As such, the process of transmitting a signal in all possible Tx directions by the BS 110 at least once is called a beam sweeping process, and the process of selecting an optimal Tx/Rx beam direction by the MS 130 is called a beam selection process. This optimal DL Tx/Rx beam selection process may be equally applied even to an UL Tx/Rx process of transmitting data from the MS 130 to the BS 110.
FIG. 2 illustrates a beam width, an elevation angle, and an azimuth in a mobile communication system using beamforming.
It will be assumed in FIG. 2 that a BS 210 is installed in a location, for example, on a building, having a height 201 from the ground, and has a predetermined beam width 205. The beam width 205 of the BS 210 may be defined for each of the elevation angle and the azimuth. Generally, the elevation angle refers to an angle (for example, an angle between an antenna and the ground) at which an antenna for transmitting and receiving radio waves sees the satellite. In the example of FIG. 2, since an antenna of the BS 210 looks down at the ground, its elevation angle 203 may be construed as an angle between a Tx beam and the vertical surface of the building on which the BS 210 is installed. Although not illustrated in FIG. 2, the azimuth may be construed as an angle of the horizontal direction in which the Tx beam is propagated.
FIG. 3 illustrates the number of Tx beams that can be used by a BS depending on the elevation angle and the azimuth.
Specifically, FIG. 3 illustrates the number of Tx beams that can be transmitted by a BS 310, under the assumption that the BS 310 is installed on, for example, a building like in FIG. 2, and the BS 310 is installed at the height of, for example, 35 m, and transmits a Tx beam having a beam width of 5° with respect to each of the elevation angle and the azimuth in one sector having an angle of 30° and coverage of 200 m.
In the example of FIG. 3, since the number of Tx beams that can be transmitted by the BS 310 is a product of 16 elevation-angle Tx directions in units of 5° and 6 azimuth Tx directions in units of 5° for each elevation-angle Tx direction, and is 96 in total, the total number of possible Tx directions of the Tx beams is 96.
Although a Tx beam transmitted by a BS is spread in the form of a sector (or fan) when there is no obstacle, it is assumed in the example of FIG. 3 that each Tx beam reaches the ground in the form of a rectangle for the purpose of convenience. In FIG. 3, the rectangles represent 96 areas where a Tx beam having specific azimuth and elevation angle has reached the ground. The 96 Tx beams are transmitted up to the farther region as the elevation angle is greater, and the Tx beams are received in the wider region as they are transmitted far away from the BS.
A ratio written in each rectangle represents the ratio of a reception (Rx) area of the Tx beam transmitted to the location of the rectangle, to a total of 96 areas, in terms of the area. It can be understood that as illustrated in FIG. 3, even for a Tx beam having the same beam width, a Tx beam that is transmitted to the region close to the boundary area of the BS is received in a much wider area depending on the elevation angle and azimuth, compared to a Tx beam that is transmitted to the region close to the central part of the BS. Simulations show that in the example of FIG. 3 where the BS's height of 35 m and the coverage of 200 m are considered, there is an area difference of a maximum of 480 times in Rx areas of a Tx beam.
If a Tx beam having the elevation angle and azimuth of a narrow beam width illustrated in the example of FIG. 3 is used, a plurality of possible Tx beams and Rx areas exist in the BS. Particularly, if a DL synchronization channel and broadcast control channel, which are transmitted by a beam sweeping scheme, are transmitted using a Tx beam having a narrow beam width as in the example of FIG. 3, each of them are repeatedly transmitted in all Tx beam directions of the narrow beam width in the BS at least once, altogether at least 96 times. Since the number of transmissions required to transmit the DL synchronization channel and broadcast control channel by the beam sweeping scheme is proportional to the number of Tx beams available in the coverage of the BS, the simplest way to reduce the Tx overhead of the DL synchronization channel and broadcast control channel in the BS of FIG. 3 is to support the full coverage of the BS with the smaller number of Tx beams. To this end, each Tx beam may need to have a wider beam width.
Generally, however, as a beam width of a Tx beam is wider, its beamforming effects are lower in proportion thereto. Conversely, as a beam width is narrower, the beamforming effects are higher. If a beam width is reduced to increase the beamforming effects, the number of Tx beams to support one BS area increases according thereto, causing an increase in the overhead associated with transmitting the broadcast-type channels. As such, the beamforming effects and the broadcast channel transmission overhead have a trade-off relationship with each other.
In order to effectively solve these and other problems and disadvantages, a new scheme is generally used, that makes a beam width used to transmit broadcast channels different from a beam width used to transmit user data. For example, a Tx beam having a beam width of 30° may be used as a Tx beam for transmitting broadcast channels in a sector having a beam width of 60°, and a Tx beam having a beam width of 10° may be used as a Tx beam for transmitting user data. In this scheme of using, a plurality of different beam widths, a Tx beam having a wide beam width is called a wide beam or a coarse beam, while a Tx beam having a narrow beam width is called a narrow beam or a fine beam.
Generally, a narrow Tx/Rx beam has a high antenna gain, but may not ensure the communication performance due to its narrow beam width if the Tx/Rx beam deviates from its direction. In addition, in the case of the narrow Tx/Rx beam, since its Tx/Rx range is limited, link fragility may occur in which the communication is cut off instantly, if a reflector or an object that is difficult to penetrate, is present between a Tx beam and a Rx beam.